1. Field of the Invention
The present invention relates to a filtering apparatus and method thereof, and more particularly to a post-filtering apparatus and method thereof for reducing coding noise without distorting a speech signal in a Modified Discrete Cosine Transform (MDCT) domain.
2. Description of the Related Art
To transmit and process a speech signal, an analog speech signal is generally subjected to a series of modulation processes, such as sampling, quantization, etc. However, since such a modulated signal is too large, there is a limit in directly processing the modulated signal. Accordingly, various codecs have been proposed for compressing and decompressing the signal.
A narrowband codec capable of encoding and decoding speech having a bandwidth of 300 Hz˜3,400 Hz exhibits a high compression ratio based on Code Excited Linear Prediction (CELP) which models a speech production process. Meanwhile, a wideband codec capable of encoding and decoding speech having a bandwidth of 50 Hz˜7,000 Hz has recently been developed to improve naturalness and articulation which are pointed out as drawbacks of the narrowband codec. As an example of the wideband codec, there are G.729.1, Adaptive Multi-Rate Wideband (AMR-WB), etc. Generally, the wideband codec transforms the signal of a time domain to that of a Modified Discrete Cosine Transform (MDCT) domain and quantizes it.
When a codec of a low bit rate is used in encoding and decoding speech, the quality of speech is degraded due to coding noise. To solve this problem, the following two methods have been proposed.
One is a method of shaping a coding noise spectrum in an encoder. In this method, the coding noise spectrum is shaped depending on a speech spectrum so that a ratio of speech signal to coding noise power in each frequency is higher than a minimum value. This method is used in CELP, Adaptive Predictive Coding (APC), Multi-Pulse Linear Predictive Coding (MPLPC), etc. Further, this method is based on a principle that a masking effect prevents humans from hearing the coding noise.
The other is a method of using an adaptive post-filter in a decoder. In this method, a filter having a frequency response similar to speech is used to reduce coding noise. Further, this method is used in 8 kb/s Vector Sum Excited Linear Prediction (VSELP), 6.7 kb/s VSELP (Japanese digital cellular, JDC), G.729B, etc.
In particular, a wideband processing post-filter has been introduced to cope with a recently increasing trend of using the wideband codec to provide higher quality of speech. As a representative example, there is an MDCT based post-filter as employed in G.729.1. This technique is based on applying the post-filter to an MDCT coefficient obtained by dequantization in the decoder, in which 160 MDCT coefficients are allocated to 10 subbands and envelopes are summed for each of the subbands. At this time, a new MDCT coefficient can be obtained by multiplying a filter coefficient based on an envelope by a filter coefficient based on the sum of the envelopes.
However, such a conventional method has a problem of distorting the speech spectrum since only the current MDCT coefficient is used. For example, if the current MDCT coefficient is small, even though a previous MDCT coefficient is large, it is necessary to allocate a small value to the current MDCT coefficient. However, the conventional method is not performed in this manner. Further, since a speech signal is linearly emphasized according to the magnitude of the speech spectrum in a section where the speech spectrum is high, the conventional problem causes sever distortion of the speech signal.